In the prior art, a honeycomb core consisting of cavities (chambers) enclosed by walls, the cavities having a triangular, quadrilateral, hexagonal (honeycomb), octagonal or circular planar shape, is used as a panel structure due to having light weight and good rigidity, but a honeycomb core is also known to have sound absorbing properties (rectifying effects). Conventionally, it has been proposed to arrange a porous sound absorbing material directly on the front surface of a honeycomb core to form a sound absorbing structure, thereby improving the high-frequency sound absorbing properties of the porous sound absorbing material up to the medium-frequency region due to the rectifying effects of the honeycomb core and the effects of a rear air layer. However, according to this structure, in a low-frequency region of 500 Hz or below, improved sound absorbing properties are not obtained unless the honeycomb core is set to a thickness of 50 mm or greater, and hence the sound absorbing structure becomes heavy and cannot be used in narrow spaces.
Furthermore, it has been proposed that a sound absorbing panel be composed by bonding and integrating a sheet-shaped body and a honeycomb core. However, this structure has inferior properties as a sound absorbing panel, and is used principally as a sound insulating material.
Moreover, it has also been proposed to install an installation section in a frame shape about the periphery of a sheet-shaped body via a vibrating attenuation member (damping material), and to fix this frame to a rigid sheet member. However, this structure does not achieve good sound absorbing properties in a low-frequency region at 500 Hz or below, and has a large thickness.
Moreover, a method has also been proposed in which a honeycomb core is not employed, but rather an organic hybrid sheet (chemical composite comprising organic origomer material dispersed in matrix polymer) is applied to create 75 to 150 mm-square airtight spaces, and is caused to perform membrane oscillation. However, since a honeycomb core is not used inside this airtight space, then it is not possible to efficiently obtain a thin sound absorbing material having a high sound absorbing rate.
Of these structures, that which employs a honeycomb core involves bonding and integrating a sound absorbing material and a sheet-shaped body via a frame, which requires complex manufacturing steps; such a structure is directed to large sound absorbing panels, and proves to be expensive and heavy for small sound absorbing materials. Furthermore, if a honeycomb core is formed by extrusion molding, then the costs of the die are high and production costs rise. Moreover, a conventional sound absorbing structure has high rigidity in the honeycomb core, and therefore if there are slightly curved portions in the installation surface, the structure cannot readily follow the installation surface and gaps occur with respect to the installation surface.
In this way, a conventional sound absorbing structure has poor sound absorbing properties in a low-frequency region of 500 Hz or below, is large in size, has drawbacks in terms of cost and weight, and is difficult to install satisfactorily if there is curvature in the installation surface.
Furthermore, for the sound absorbing material, a porous sound absorbing material such as polyurethane foam, glass wool, nonwoven fabric, felt, or the like, is used. A porous sound absorbing material has high sound absorbing properties in respect of noise centered in a high-frequency region, but does not achieve good sound absorbing properties in respect of noise centered in a medium-frequency region, unless formed to a large thickness of 50 mm or greater, and therefore has low sound absorbing effects at normal thicknesses.
Therefore, a sound absorbing structure for medium and high-frequency regions has been proposed in which the apertures of a plurality of substantially tubular spaces (absorbing cavities) are covered with a sound absorbing material, such as a porous material, in order to improve sound absorbing properties in the medium-frequency region. Moreover, a sound absorbing material made of such as nonwoven cloth with regulated air permeability, and a sound absorbing sheet in which a nonwoven cloth with regulated air permeability, and the like, is layered onto the surface of a base material via an air layer, and the like, have also been proposed. However, in these sound absorbing materials, the thickness of the sound absorbing material must be made large in order to raise the sound absorbing rate in the medium-frequency region, and therefore countermeasures to noise are difficult to achieve in locations where, for instance, there is restricted space for installing sound absorbing material.
Furthermore, there has also been a proposal for an air sound absorbing member which has generally higher overall sound absorbing properties than a conventional resonance absorber in a range from a medium-frequency region to the high-frequency region, from approximately 400 Hz to approximately 10,000 Hz, in which a porous layer is provided on a plastic resonance absorber having a hollow chamber, the porous layer being provided separately from the hollow chamber. However, in the air noise absorbing member, the sound absorbing rate of the plastic resonance absorber (sound absorbing structure) is low, and furthermore because the covering porous layer does not make contact with the membrane oscillation body of the sound absorbing material, then the thickness of the air sound absorbing member becomes large.